[AMDGPU] Strengthen preload intrinsics to noundef and nonnull (#92801)

The various preloaded registers (workitem IDs, workgroup IDs, and
various implicit pointers) always have a finite, invariant,

[AMDGPU] Strengthen preload intrinsics to noundef and nonnull (#92801)

The various preloaded registers (workitem IDs, workgroup IDs, and
various implicit pointers) always have a finite, invariant, well-defined
value throughout a well-defined program.

In cases where the compiler infers or the user declares that some
implicit input will not be used (ex. via amdgcn-no-workitem-id-y), the
behavior of the entire program is undefined, since that misdeclaration
can cause arbitrary other preloaded-register intrinsics to access the
wrong register. This case is not expected to arise in practice, but
could occur when the no implicit argument attributes were not cleared
correctly in the presence of external functions, indrect calls, or other
means of executing un-analyzable code. Failure to detect that case would
be a bug in the attributor.

This commit updates the documentation to reflect this long-standing
reality.

Then, on the basis that all implicit arguments are defined in all
correct programs, the intrinsics that return those values are
annototated with `noundef``. Some implicit pointer arguments gain a
`nonnull`, but the kernel argument segment pointer or implicit argument
pointers don't necessarily have this property.

This will prevent spurious calls to `freeze` in front-end optimizations
that destroy user-provided ranges on built-in IDs.

(While I'm here, this commit adds a test for `noundef` on kernel
arguments which is currently unimplemented)

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[AMDGPU] Add IR lowering changes for preloaded kernargs

Preloaded kernel arguments should not be lowered in the IR pass
AMDGPULowerKernelArguments. Therefore it's necessary to calculate the
total nu

[AMDGPU] Add IR lowering changes for preloaded kernargs

Preloaded kernel arguments should not be lowered in the IR pass
AMDGPULowerKernelArguments. Therefore it's necessary to calculate the
total number of user SGPRs that are available for preloading and how
many SGPRs would be required to preload each argument to determine
whether we should skip lowering i.e. the argument will be preloaded
instead.

Reviewed By: bcahoon

Differential Revision: https://reviews.llvm.org/D156853

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